Go with the second one - but you will probably have to dismantle it and drill out the inlet holes as they are very small and you will need the gas flow to make them work well.

The phrase air ram means that is limited to compressed air, meaning it will not be rated much above 10bar so you will have to build a low pressure system.

Using CO2 will cause the rams to cool significantly but you will have to live with it - you may find that on cooling the alumunium body will cool more than the steel drive rod and this can cause the ram to seize up.

Trev